FIG. 5 shows an example of a conventional resin sealed semiconductor device assembly configuration. The illustrated configuration includes a metallic base plate 1 that also serves as a heat radiation plate, a semiconductor element 2 loaded or placed on the metallic base plate 1 via a substrate 3. A lead-out terminal 4 is soldered to a surface electrode of the semiconductor element 2 and to a conductor pattern of the substrate 3 and is drawn out and upward. A molded resin covering case 5 is bonded with an adhesive 6 to a peripheral edge of metallic base plate 1, while a molded resin terminal-supporting plate 7, that also serves as the case lid on an upper surface of covering case 5, is held in place by a sealing resin 8 that fills the inside of covering case 5.
Two different types of sealing resin 8 can be used: one is made entirely of epoxy resin; and the other is divided into two layers (upper and lower), wherein the lower layer is made of silicon gel and the upper layer is made of epoxy resin. In the two-layered (upper and lower) sealing resin 8, as the cross-sectional view of the main section in FIG. 6 shows, a step difference 5a is usually prepared on an inner surface of covering case 5, and filling of the inside of covering case 5 with silicon gel 8a starts and stops before the level of silicon gel 8a reaches the step difference 5a. The remaining portion is filled with epoxy resin 8b to harden inside of the covering case 5.
Although the type of-resin material for covering case 5 and terminal-supporting plate 7 are selected in consideration of their resistance to heat, flameproofing property, coloring property, electric characteristics, and molding costs, usually PBT (polybutylene terephthalate) resin or PPS (polyphenylene sulfide) resin are used for covering case 5 and PPS resin for terminal-supporting plate 7.
A problem exists with the conventional resin sealed semiconductor device illustrated in FIG. 5, namely, peeling occurs between the structural components by mutual adhesive failure of the components during actual use or in temperature cycle tests. The peeling further causes trouble such as the degradation of the moisture resistance performance of the device and leaking of the sealing resin (gelatinoid filler) from the covering case. In particular, the adherent surface between the covering case and the sealing resin, and the adherent surface between the terminal-supporting plate and the sealing resin easily peels off.
Further, epoxy resin is used for the sealing resin and solidified at about 135.degree. C. The transition temperature required to solidify epoxy resin into glass is 140.degree.-160.degree. C., and at temperatures below that range the elasticity of the hardened epoxy resin is highly stable at approximately 1,200 kg/mm.sup.2. The glass transition temperature for PPS resin is 80.degree.-90.degree. C., and although at temperatures below this range the elasticity of PPS resin is very stable at approximately about 1,800 kg/mm.sup.2, at temperatures above this range the elasticity of the PPS resin tends to gradually decrease. Accordingly, when a covering case and terminal-supporting plate made of PPS resin, in order to fill the inside of the covering case and solidify the PPS resin at about 135.degree. C., since the elasticity of the PPS resin, which decreases somewhat, does not change considerably, a difference in the elasticity of the PPS resin and the epoxy resins arises, causing further latent stress in the adherent surface between the covering case and the sealing resin, and between the terminal-supporting plate and the sealing resin.
Since an external mechanical stress such as high tension or bending load is applied to said adherent surfaces during connection of the external wiring to lead-out terminals, peeling occurs between the terminal-supporting plate, which supports the lead-out terminals, and the sealing resin, and further between the covering case and the sealing resin. When the lead-out terminal comes loose due to the peeling thus caused, stress is also applied to the soldered sections between the lead-out terminals and the semiconductor device, thereby causing serious defects such as damage to the semiconductor device.
Furthermore, when an attempt is made to use an adhesive to solidify the adhesive used to bond the metallic base plate to the external covering case, the adhesive runs out of the adherent section and then out from under the lower surface of the metallic base plate and adheres thereto, thereby damaging the smoothness of the mounting surface, reducing the adhesiveness of the semiconductor device to an apparatus, degrading the heat radiation performance of the semiconductor device, or causing the adhesive to further run into the screw holes prepared on the metallic base plate for mounting the semiconductor device, thereby disabling the mounting of the semiconductor device, or loosening the screws in use.
Given the above situation, the object of the present invention is to provide a stiff and highly reliable resin sealed semiconductor device by resolving said problems, and by preventing the adhesive between structural components to lose its adhesive properties.